1. Field of the Invention
The present invention relates in general to generating a frequency modulated UHF radio transmission signal, and more particularly to the direct, oscillator generation of a frequency modulated radio transmission signal.
2. Description of the Prior Art
Radio signals in the frequency range of 400 megahertz ("MHz") to 1,000 MHz are used for a variety of purposes, such as fixed, mobile, maritime, aeronautical, and amateur transmissions, for radio and television broadcasting, and for radio navigation. This interval of radio frequencies occupies a portion of a larger interval of frequencies known as the ultrahigh frequencies ("UHF") which extend from 300 MHz to 3,000 MHz.
One of the uses made of radio transmissions in the UHF range is that of linking a studio of a radio station with the station's remote transmitter site, i.e. a studio transmitter link, and for relaying radio broadcasting signals between cities, i.e. an inter-city link. On such a studio transmitter link or an inter-city link, the radio broadcast signals are frequently transmitted as a frequency modulated ("FM") UHF radio signal. Maintaining the quality of a radio broadcast signal during its transmission over a studio transmitter link or an inter-city link as a FM UHF signal requires stringent standards on the quality of the transmitted signal and, hence, on the transmitter which generates the signal. Furthermore, improvements in the audio fidelity of radio broadcast signals, such as those occasioned by the recent introduction of optical disk digital recording and reproduction technology, increase the demand for higher quality in the signal transmitted over these links. The prior methods for generating these FM UHF radio signals have been inadequate to satisfy the increasing requirements for high quality transmissions.
One of the previous methods for generating FM UHF radio signals was to generate a FM signal with an oscillator at a frequency significantly lower than the transmission frequency. Such a FM signal was generated at a frequency below 60 MHz. The range of frequencies for the FM signal is known as the very high frequency ("VHF") band. In this example, if a UHF transmission frequency of 960 MHz were desired, then the VHF signal modulated at a frequency of 60 MHz would be multiplied by a factor of 16 to increase the frequency to that desired for the UHF transmission. Because of the frequency multiplication requirement, the quantum of the FM signal with respect to the VHF signal would be a fraction, i.e. 1/16 of the FM signal required for the UHF radio signal.
The preceding frequency multiplying technique for generating FM UHF radio signals presents several problems. Since the FM signal generated by the oscillator is frequency multiplied by a relatively large factor, such as 16, any instability in that signal is also amplified by that same factor. Consequently, the technique for generating UHF radio signals requires stringent standards on the frequency stability of an oscillator generating the VHF signal. Similarly, any noise in the FM signal is also amplified by the multiplying factor. Thus, stringent noise requirements must be met by the oscillator generating the VHF signals. Furthermore, it is difficult to design electronic circuits, such as filters, for adequately performing the necessary frequency multiplication to produce a high quality signal without spurious signals at a UHF radio frequency. Only a relatively ow power signal is produced by frequency multiplication.
The low power signal requires a comparatively large number of power amplifying stages to reach a useful power level for transmitting the FM UHF signal. Finally, it is more costly to design power amplifiers at ultrahigh radio frequencies which have numerous successive amplifying stages, because inter-stage signal coupling tends to make them oscillate uncontrollably.